Offset lithography is a common method utilizing in modern printing operations. (Note that for the purpose hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process, a printing plate (i.e., which may be a flat plate, the surface of a cylinder, belt, etc.) can be configured with “image regions” formed of, for example, hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. Such image regions correspond to the areas on the final print (i.e., the target substrate) that are occupied by a printing or a marking material such as ink, whereas the non-image regions correspond to the areas on the final print that are not occupied by the marking material.
The Variable Data Lithography (also referred to as Digital Lithography or Digital Offset) printing process begins with a fountain solution used to dampen a silicone imaging plate on an imaging drum. The fountain solution forms a film on the silicone plate that is on the order of about one (1) micron thick. The drum rotates to an “exposure” station where a high power laser imager is used to remove the fountain solution at the locations where the image pixels are to be formed. This forms a fountain solution based “latent image.” The drum then further rotates to a “development” station where lithographic-like ink is brought into contact with the fountain solution based “latent image” and ink “develops” onto the places where the laser has removed the fountain solution. The ink is hydrophobic. An ultra violet (UV) light may be applied so that photo-initiators in the ink may partially cure the ink to prepare it for high efficiency transfer to a print media such as paper. The drum then rotates to a transfer station where the ink is transferred to a printing media such as paper. The silicone plate is compliant, so an offset blanket is not used to aid transfer. UV light may be applied to the paper with ink to fully cure the ink on the paper. The ink is on the order of one (1) micron pile height on the paper.
The formation of the image on the printing plate is done with imaging modules each using a linear output high power infrared (IR) laser to illuminate a digital light projector (DLP) multi-mirror array, also referred to as the “DMD” (Digital Micromirror Device). The mirror array is similar to what is commonly used in computer projectors and some televisions. The laser provides constant illumination to the mirror array. The mirror array deflects individual mirrors to form the pixels on the image plane to pixel-wise evaporate the fountain solution on the silicone plate. If a pixel is not to be turned on, the mirrors for that pixel deflect such that the laser illumination for that pixel does not hit the silicone surface, but goes into a chilled light dump heat sink. A single laser and mirror array form an imaging module that provides imaging capability for approximately one (1) inch in the cross-process direction. Thus, a single imaging module simultaneously images a one (1) inch by one (1) pixel line of the image for a given scan line. At the next scan line, the imaging module images the next one (1) inch by one (1) pixel line segment. By using several imaging modules, comprising several lasers and several mirror-arrays, butted together, imaging function for a very wide cross-process width can be achieved.
One non-limiting example of a DMD system utilized in the context of a lithographic application is disclosed in U.S. Pat. No. 8,508,791 entitled “Image feedforward laser power control for a multi-mirror based high power imager” which issued to Peter Paul et al on Aug. 13, 2013, and is assigned to Xerox Corporation of Norwalk, Conn. U.S. Pat. No. 8,508,791 is incorporated herein by reference in its entirety.
Some laser imaging systems utilize high intensity line-source lasers to deliver energy selectively to clear thermo-chromic inks. The thermo-chromic inks change from clear to black at a given energy level. Digitally selective heating of the inks with an LIM (Laser Imaging Module) delivers a digital image on a surface pre-coated with the thermo-chromatic inks. DMD's were originally designed to operate with lower incident energy levels. The current level of incident energy exceeds the device's ability to adequately dissipate heat from the DMD chip. If the DMD chip (and its internal components) exceeds a critical temperature (e.g., 70° C.), the DMD chip can become damaged. Currently the DMD can only run at 80% power for 20 seconds before reaching a temperature maximum. The desire is to have the ability to operate the device at 100% power at 100% duty-cycle.